1. Field of the Invention
The present invention relates to microlenses. More specifically, the present invention relates to cylindrical microlenses for use with laser diodes, laser diode bars, and integrated optics.
2. Description of Related Art
A lens is an optical element that can focus or de-focus electromagnetic radiation (i.e., light). The most common types of lenses are spherical; for example, a circular convex lens focuses light to a point. Such lenses are useful in many applications such as imaging, photolithography, and metrology. The common spherical lens has a shape that is symmetric about an optic axis.
Another lens that is important is a cylindrical lens. A cylindrical converging lens focuses light along a line, typically termed the xe2x80x9cline focus.xe2x80x9d The typical cylindrical lens is shaped symmetrically around a principal axis, which is orthogonal to the optic axis. For example, a cylindrical lens may have the shape of a cylinder, with circular dimensions around a central axis. Light is made incident on a first curved surface of the cylinder, and exits from the other second curved side of the cylinder.
For many applications, a circular cross-section is undesirable, and therefore, the curves of cylindrical lenses may require specific shapes that differ from the circular curve of the previous example. The required shape may be flat or it could be some other non-circular curve such as an ellipse of hyperbola. In other words, cylindrical lenses may be formed with a variety of curved surfaces. The exact shape is highly dependent upon the application. For example, laser diodes and similar types of architecture have different divergence angles along orthogonal axis due to their rectangular shaped output apertures. Because of this characteristic, a cylindrical lens can be shaped to collimate the fast axis (i.e., the fast divergent axis) of a laser diode by matching the divergence of the slow axis (i.e., the slow divergence axis). One method for creating cylindrical lenses with selected shapes is described and claimed in U.S. Pat. No. 5,080,706 issued Jan. 14, 1992 and U.S. Pat. No. 5,081,639 issued Jan. 14, 1992, each issued to Snyder et al., and assigned to the assignee of the instant application. In addition, the method for creating cylindrical lenses with selected shapes is described in xe2x80x9cFast diffraction-limited cylindrical microlenses,xe2x80x9d by Snyder et al., Applied Optics Vol. 30, pp. 2743-2747, 1991.
Cylindrical microlenses shaped for a specific application are utilized for integrated optics and optically conditioning radiation of laser diode bars. In recent years, the ability to package and to condition the radiance of laser diodes using shaped-fiber cylindrical-microlens technology has dramatically increased the number of applications that can be practically engaged by diode laser arrays. Government research and development in this area has created improvements in this technology in an effort to supply large radiance conditioned laser diode array sources for its own internal programs as well as for industrial applications.
Original efforts on the development of modular integrated laser diode packaging technology is described in xe2x80x9cApplications of Micro-lens-Conditioned Laser Diode arrays,xe2x80x9d by R. J. Beach et al., SPIE Vol. 2383, p 283, 1995 and in U.S. Pat. No. 5,105,429 issued Aug. 14, 1992 to Mundinger et al. Recently, advances beyond the original rack and stack technologies in which typically only a single laser diode bar was attached to a single high performance heat sink have enabled monolithic laser diode packages in which multiple diode bars are attached to a single high performance heat sink. This technology advance has led to larger laser diode arrays and larger diode-pumped laser systems. One type of monolithic package is manufactured from silicon substrates and uses microchannels fabricated directly into the silicon to aggressively remove the waste heat that is generated by diode bars that are attached to the silicon. This type of package, which utilizes Silicon Monolithic Microchannels (i.e., SiMM) was originally intended for high average power applications. There is also a low duty factor package, which is closely related to the SiMM package, but does not incorporate microchannels in the silicon. This package is known as V-BASIS, and except for the lack of microchannels, is very similar to the SiMM package in its structure. Basically, the SiMM package retains many of the same basic features of the original rack and stack package, but engages a higher level of integration with multiple diode bars attached to a single based chip carrier. Such approaches are described and claimed in U.S. Pat. No. 5,548,605 issued Aug. 20, 1996 to Benett et al., U.S. Pat. No. 5,828,683 issued Oct. 27, 1998 to Freitas, and U.S. Pat. No. 5,923,481 issued Jul. 13, 1999 to Skidmore et al., and assigned to the assignee of the instant application.
Both the SiMM package and the V-BASIS package have been remarkably successful in building very large laser diode arrays. However, a major problem remains with conventional optical conditioning of the radiation emitted by laser diode bars attached to these packages if conventional microlenses are used. The problem is associated with the V-grooves in which the laser diode bars are attached. Due to the orientation of the V-grooves, the radiation emitted by laser diodes that are attached to them is directed away from the normal to the face of the package. In some instances, this off-normal directed emission is an aspect of the package that can be overcome by suitably orienting the package to compensate for the off-normal emission direction. In other instances, it is required that the conditioned light be emitted from the package in a direction along the normal to the face of the package. The V-grooves result from the etching process used in their fabrication. In addition to the formation of V-grooves for positioning and mounting laser diode bars, the same V-groove technology is used to fabricate microchannels into the silicon substrate. Anistropic etching of silicon takes advantage of the fact that some chemicals, e.g., potassium hydroxide, etch crystal planes of different orientations at different rates. In  less than 110 greater than  oriented wafers, (the surface of the wafer is a  less than 110 greater than  plane), etch rate differences can be exploited to etch channels that are perpendicular to the surface of the wafer. This is accomplished by creating a mask on the surface of the wafer that is aligned with the  less than 111 greater than  planes on the wafer. When etched, these slow-etching, perpendicular  less than 111 greater than  planes then become the walls of the channels. With the appropriate angular orientation of an etch mask on a  less than 110 greater than  oriented silicon wafer, the result of the above etching method is to produce V-grooves wherein laser emitting diodes or laser diode bars are attached to the slanted surfaces, i.e., the  less than 111 greater than  plane, and as such are oriented in a very specific way relative to the  less than 110 greater than  normal direction.
Accordingly, in addition to performing the required lens collimation task, the present invention provides a cylindrical microlens for deviating the off-normal optical rays to enable emission which is normal to the plane of the array. In addition, the present invention can be incorporated into high average power, high density, two-dimensional arrays to solve a need in industry and research environments for optical conditioning of these devices.
Accordingly, the present invention provides a lens, which includes an internally reflecting surface, which functions to deviate the direction of light that enters the lens from its original propagation direction while providing a collimated output.
Another aspect of the present invention is to provide a laser diode apparatus whereby a fast (high numerical aperture) cylindrical microlens conditions the output from a laser diode and deviates emission from the laser diode from its original direction in such a way that the laser diode emission is directed normal to a diode array plane.
A further aspect of the present invention is to provide a two-dimensional array of laser diode bars wherein an output emission from each laser diode bar is conditioned by a fast cylindrical microlens and deviated from its original direction in such a way that the laser diode bar output emission is directed normal to a diode bar array plane.
The present invention additionally provides for a method of conditioning emission from a laser diode so that the output emission from the laser diode is deviated from its original direction and directed normal to a diode array plane.
Cylindrical microlenses have found wide application as optical devices for collimating the fast-axis radiation from laser diode emitters (i.e., the direction perpendicular to the p-n junction of the diode). When microlens conditioning is applied to large two-dimensional semiconductor laser diode arrays consisting of multiple diode bars, the effective radiance (W/cm2-str) of the array is increased by the ratio of the fast-axis divergence of the diode radiation before the cylindrical microlens lens to the fast-axis divergence of the radiation after the cylindrical microlens. This radiance enhancement, which can be up to one hundred times, has many practical applications and has tremendously enhanced the utility of laser diode arrays.
The present invention includes a totally reflecting internal surface in a cylindrical microlens, and is useful in applications where radiance enhancement is advantageous. Additionally, by including a totally reflective internal surface in the lens structure itself, the present invention enables the direction of the emitted light from the microlens to be deviated from its original optic axis in a precisely controlled way. Particularly for laser diode array packaging technologies in which the diode radiation is emitted in some direction not normal to the plane of the diode package, this feature of deviating the direction of the light while also collimating it with a single structure broadens the application space of laser diode arrays.
Finally, the lens of the present invention can be used to optically condition laser diode arrays to provide directional and narrow light sources such as are required in various spectroscopic devices used to identify chemical and biological substances.